We are continuing to use the rat as a model to study the biosynthesis and metabolism of plasma apolipoprotein B (apoB), the structural protein of very low density lipoproteins (VLDL), and apoA-I, the major protein of high density lipoproteins (HDL). Evidence has been obtained that rat livers produce three forms of apoB, differing in their apparent molecular weights. Furthermore, the relative amount of the three forms produced appears to be regulated by diet and perhaps other physiological variables. This seems important because in other studies we have shown that all three forms of apoB are independently metabolized, the relative circulating lifetimes being as follows: B-95 greater than B-100 greater than B-48. Together, these findings point to a complex mechanism in the rat whereby the metabolic fate of hepatic VLDL and its lipid component is physiologically regulated by the form of apoB it contains. Amino-terminal amino acid analysis of the two major apoA-I isoforms from rat plasma revealed that the more acidic isoform is a proform containing a 6-amino acid extension at the NH2-terminus. Similarly, the more basic isoform was identified as mature apoA-I. Both rat liver and small intestine synthesize and secrete in vivo predominantly proapoA-I, which is slowly processed in the circulation to mature apoA-I (about 80% complete in 10 h). Slow and incomplete processing (about 20% complete in 10h) occurs in whole, heparinized blood or in plasma incubated at 37 C in vitro. Thus, while the protease presumably required for the conversion is present in plasma, it may be labile. The protease also exhibits an apparent requirement for divalent metal. Additionally, circulating rat proapoA-I, like mature apoA-I, is virtually all associated with lipoproteins.